An image forming apparatus having a copier function and a printer function includes a paper feeding portion that stacks and contains paper sheets (sheets) used for forming images, a transferring portion that transfers a toner image formed on a photoconductor drum to a paper sheet, and a paper carrying apparatus for carrying paper sheets from the paper feeding potion to the transferring portion.
The paper carrying apparatus includes a plurality of sheet carrying mechanisms (roller pairs) capable of pinching and sending out paper sheets, which are located at predetermined intervals along a paper carrying path, and carries a paper sheet by relaying a paper sheet from the roller pair to the next pair. The paper carrying apparatus causes the upstream roller pair and the downstream roller pair to operate at the same paper carrying velocity to smoothly carry paper sheets.
In an image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 11-124255, a stepping motor is used to drive resist rollers, which are roller pairs for supplying paper sheets to the transferring portion.
It is assumed here that a stepping motor is used to drive the downstream roller pair and that a circumferential velocity (paper carrying velocity) of the upstream roller pair is reduced since a roller radius of the upstream roller pair is reduced due to the worn-out with the passage of time. In this case, while the upstream roller pair and the downstream roller pair carry the same paper sheet, the paper sheet is stretched by the both roller pairs, and when the rear end of the paper sheet subsequently passes through the upstream roller pair, the load torque for the downstream roller pair is abruptly reduced, and there's a fear that the stepping motor driving the downstream roller pair steps out or stops.
If the motor stops while carrying a paper sheets, this is determined as jamming, resulting in turning-off of a drive current to a drive source of the entire image forming apparatus. Although the normal operation may be performed after the jamming disposal operation if the drive current is resupplied to the apparatus (the apparatus is restarted), the jamming disposal operation must be performed for the paper sheet that was halfway carried.